Digital watermarking systems are known in the art, and are shown, e.g., in application Ser. Nos. 09/503,881 (now U.S. Pat. No. 6,614,914) and 09/452,023 (now U.S. Pat. No. 6,408,082).
Steganographic calibration signals (sometimes termed “orientation signals,” “marker signals,” reference signals,” “grid signals,” etc.) are sometimes included with digital watermarking signals so that subsequent distortion of the object thereby marked (e.g., a digital image file, audio clip, document, etc.) can later be discerned and compensated-for. Such arrangements are detailed in the cited applications.
Systems for detecting watermarks from marked objects sometimes fail if the image object encompasses several separately-watermarked components (e.g., a scanned magazine page with several different images, or photocopy data resulting from scanning while several documents are on the photocopier platen, as shown in FIG. 1). Each component may include its own calibration signal, confusing the detection system.
In accordance with certain embodiments of the present invention, this problem is addressed by a proximity-based approach, and/or a multiple grids-based approach. In accordance with other embodiments of the present invention, the calibration signal can—itself—convey the watermark information, so it serves both a calibration and a payload-conveyance function.
The foregoing and additional features and advantage of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.